@ThomasP my understanding is that it was 7075 not Alu 7076 but those are almost the same material anyway. Minor quibble on my part but totally agree with everything you wrote. EDIT: I misread and you did write that 7076 is essentially the same thing as ESD whereas 7075 was introduced by Japan in 1943. Mind blowing info. Thanks!
Regarding the primary discussion topic, aerodynamics, I believe the main things that we're all talking about are the zero-lift drag coefficient and wing area of an aircraft. I only have a low-level, mechanical understanding of these principles of flight, but a quick summary for WW2 aircraft is as follows:
In general, the major aerodynamic variables for drag are the zero-lift drag coefficient and the wing area. If you have both, then you can calculate how aerodynamically efficient an air frame is.
Japanese aircraft tended to have particularly good streamlining, but then they also had big wings relative to horsepower and weight, which watered down their excellent streamlining.
But this is all speculation as I have never seen a ZLDC calculated for any Japanese aircraft, not even the Zero. Although
some aviation buffs have used estimated ZLDCs which were reverse engineered from TAIC records, these are of unknown pedigree as well. This video does a really great job of reverse engineering TAIC's calculations, and some Japanese sources, to get the ZLDC for the Ki-84. But as the video pointed out, TAIC appears to have just transplanted the Ki-43's ZLDC to the Ki-84's wing area. Additionally, Nakajima burned all of their records after the war, so the final drag estimate is also just an estimate with no valid sourcing. This is definitely not the right number. And we don't actually know how the Frank's aerodynamic efficiency was.
Just a side note: I'd love to see the zero lift drag coefficient calculated for the N1K2-J and the N1K1-J but my guess is that it doesn't exist. But because it has laminar airfoils and a large propeller, it's a good indicator of Japanese aerodynamic technology.
The Japanese were early adopters of "laminar" flow airfoils. Kawanishi was the first company to fly such an aircraft outside of the US, but the J2M1 Raiden also had a semi-laminar airfoil (according to Japanese Wikipedia, which sources Maru Mechanic IIRC for this). However, except for the Suisei dive bomber and the Ki-61, all Japanese aircraft were relatively high drag radial aircraft, with extra emphasis spent on streamlining to compensate for their lower horsepower output as well as extensive hand finishing using machinists.
The Mustang (D model), as far as I'm aware, has the lowest Zero-Lift Drag Coefficient out of all aircraft produced in large quantities during the war. I think much of that is because it uses a liquid-cooled engine. The Ki-64 probably had an overall lower zero-lift drag coefficient though but that was a prototype.